Three commercial nanofiltration membranes were studied: two negatively charged (NF 40 and NTR 7450) and one positively charged membrane (UTC 20). The membrane pore radii were estimated using filtration experiments with uncharged solutes. For the analysis of the results the Spiegler-Kedem equation and the steric hindrance pore model were used. Salt retention measurements were carried out for NaCl, MgCl2, Na2SO4 and MgSO4 at different concentrations. The experimental results were evaluated with the Donnan exclusion theory. For both NF 40 and UTC 20 the results could not be ascribed to charge effects alone and the retention sequence of the salt solutions was found inversely proportional to the salt diffusion coefficients in water. For the NTR 7450 membrane, which had larger pores, the Donnan exclusion theory could be used and salt retention was mainly determined by charge effects. It was also shown that transport by diffusion could not be neglected.